naive implementation of 'bindsym --release' (and bindcode)

The implementation is naive because the user has to generate exactly the
event he specified. That is, if you use this binding:

    bindsym --release $mod+x exec import /tmp/latest-screenshot.png

Then it will only be triggered if you hit $mod, hit x, release x,
release $mod. It will not be triggered if you hit $mod, hit x, release
$mod, release x. The reason is that the KeyRelease event in the latter
case will not have the modifier in its flags, so it doesn’t match the
configured binding.

include/data.h

@@ -200,6 +200,10 @@ struct regex {
  *
  */
 struct Binding {
+    /** If true, the binding should be executed upon a KeyRelease event, not a
+     * KeyPress (the default). */
+    bool release;
+
     /** Symbol the user specified in configfile, if any. This needs to be
      * stored with the binding to be able to re-convert it into a keycode
      * if the keyboard mapping changes (using Xmodmap for example) */